1. Field of the Invention
The present invention relates to a digital switching system, and a method of switching data of the same. In particular, the invention is concerned with a speech path apparatus of a digital switch, and is suitable for use, for example, in case of changing a highway where data is to be accommodated.
2. Description of the Related Art
In a digital switch having a connection configuration wherein the input side of a speech path apparatus in a distribution stage is connected with a plurality of receiving (or inputting) highways, and the output side of the speech path apparatus is connected with a plurality of transmitting (or outputting) highways, connection among the plurality of either the inputting or the outputting highways is switched by controlling the speech path apparatus in response to connection information supplied from an upper layer controller. Data with a time-shared time slot as a unit of data are supplied, and outputted via the highways. Each frame making up a plurality of data units supplies L time slots accommodated therein at a frequency of, for example, 8 kHz. It follows therefore that switching of data through the intermediary of the speech path apparatus in the distribution stage corresponds to interchange of the time slots among the plurality of either the inputting or the outputting highways.
With such a digital switch as described above, an increase in the number of the highways accommodated therein results in expansion in the scale on which the time slots are interchanged, and accordingly, a primary switch (T switch)—a highway switch (S switch)—a secondary switch (T switch) are connected in multiple stages with the speech path apparatus in the distribution stage to which highway interface units are connected. Such connection results in expansion in the scale of a network. Besides, in the case where subscribers are accommodated, the speech path apparatus in the distribution stage and interface units on the side of the subscribers are connected with the digital switch.
In general, a switch is provided with a redundant highway serving as a backup in addition to its configuration made up of highways for current use so as to be able to limit a range of trouble even if, for example, the trouble occurs to any of the highways. With a digital switch provided with a countermeasure for occurrence of trouble, connection thereof with highways is switched from one of them in current use to one for use as a backup, in step with synchronization between the apparatus and the unit. By means of such switching, an operation is executed such that neither omission nor duplication occurs to data transmitted through the highways.
As described in the foregoing, the conventional digital switch adopts a three-stage configuration, excellent in respect of traffic characteristics, economics, and potential for capacity expansion, which is implemented on a single plane, allowing parallel switching to be effected, however, the primary switch and the secondary switch (T switch) as a selector need to be installed between the highway interface units and a highway switch of the speech path apparatus in the distribution stage, respectively. Such a configuration results in an increase of the scale of the system. Furthermore, since synchronization by clock is executed through the selectors, delay occurs to digital data. Consequently, a data transit time throughout the whole system is lengthened.
Further, the redundant configuration providing for the backup constitutes additional highway units. Since one or more highways are provided for as backup highways, this will result in deterioration of utilization efficiency of the resources of the conventional digital switch.